Many scientific instruments require fine positioning relative to an object to be analyzed or tested. For example, in using a scanning tunneling microscope (STM), it is necessary to be able to position the object to be scanned relative to the microscope in such a way that the microscope accurately scans the object. When the instrument operates on a very small scale, very fine positioning of the object is necessary to ensure successful operation of the instrument or examination of the object. Many times, positioning of an object within a single Angstrom range is necessary in order to achieve successful utilization of an instrument. This is most often done in the microscopic environment by placing either the microscope or the sample to be examined on a moveable mounting. The mounting is then translated in two orthogonal directions, under digital control, to position the sample relative to the microscope for examination. Accordingly, there has been significant effort to develop linear translating motors, and controllers for those motors, which are capable of operating on this scale.
One type of controller used to achieve fine adjustments of scanning tunneling microscopes is known as the "inchworm" linear translator. An inchworm translator consists of three piezoelectric components, two annular clamp components joined by a cylindrical drive component in the middle. The annular components are positioned on the axis of a shaft. This general arrangement is indicated in FIG. 1, where the two annular piezoelectric clamps are indicated at 12 and 14 and the connecting drive portion is indicated at 16. The shaft is indicated at 20. Electrical contacts are provided on the individual interior annular surfaces and the common exterior annular surface of both of the clamps 12 and 14 and the drive portion 16. The inchworm operates in a manner which gives it its name. A suitable electric field is applied to close one of the clamps, while the other is open. Then a suitable electric field is applied to elongate the drive portion between the two clamps. With the voltage held on the drive portion, the open clamp is then closed, and the closed clamp is then opened. The electric voltage on the drive portion is then released, thereby causing the drive portion to contract. Again, the voltages on the clamps are reversed and the process can be repeated. In this way, the drive portion inches down the shaft. Either the shaft or the inchworm body can be attached either to the mounting or to the sample.
Control electronics have been built which operate such inchworm motors in the prior art. Such controllers have been capable of operating an inchworm controller at high speeds, with reasonable accuracy, even with a maximum output voltage of 430 volts and with less than 0.5 millivolts of noise. Such a circuit is described by Jeon and Willis in J. Vac. Sci. Technol. A, 6:4, page 2418 (1991). For some very fine applications, even this degree of resolution and noise is insufficient, and further resolution and decrease in noise content of the driving signal is desirable.